1. Field of the Invention
The present invention relates to techniques for optimizing the number of unit cells or the electrode area thereof so as to maximize an amount of energy which the fuel cell system can output, thereby lengthening a drive time of a device to be supplied with electric power from a fuel cell system.
2. Description of Related Art
Recent advancement in functionality of portable electronic devices (for example, a notebook personal computer, a video camera, a digital camera, and a portable phone), which is accompanied by expansion of broadband, has a tendency of increase in necessary power for driving the devices.
Portable electronic devices are generally battery-operated for convenience thereof. As the enhancement of functionality and the miniaturization of portable electronic devices are advanced, increase in the energy density of batteries is demanded.
A lithium-ion secondary battery is known as one of major batteries. The volume energy density of the lithium-ion secondary battery is about 400 Wh per litter or more. However, it is said that the volume energy density of a fuel cell on which research and development are energetically conducted at present, for example, a direct methanol fuel cell using methanol as fuel is several times larger than that of the lithium-ion secondary battery. That is, the fuel cell is expected as a next-generation battery because of high energy density thereof. However, the output density per unit cell (or per single cell) of the fuel cell and the operating voltage thereof are low. Therefore, the fuel cell requires various contrivances in order to drive electronic devices.
For example, a configuration mode, in which the terminal voltage of a fuel cell is raised by a DC-DC converter to thereby generate a necessary voltage for driving an electronic device, is known (see, for instance, Japanese Patent Application Publication No. 2000-188120). The number of unit cells constituting the fuel cell is obtained by dividing the maximum power demanded of the fuel cell system by the maximum output per unit cell. The obtained number of unit cells is the minimum number of unit cells constituting the fuel cell system. Thus, miniaturization of a fuel cell can be achieved.
Also, a method for controlling what is called a hybrid fuel cell system, in which a fuel cell is combined with a rechargeable secondary battery, in such a way as to maximize an output of the fuel cell thereof has been proposed (see, for example, Japanese Patent Application Publication No. 7-153474).
Thus, the output density of the entire fuel cell system can be maximized by performing output design and control on the fuel cell in such a way as to cause the fuel cell to operate so that an output of the unit cell is close to the maximum output thereof.